The role of tissue microstructure in the tissue mechanical properties is important to understanding the determinants of skeletal integrity. The goal of this proposal is to correlate bone tissue composition with tissue mechanical properties for treatments that result in well-established alterations in bone matrix composition and altered whole bone strength to test the following hypotheses: Hypothesis 1: Reduced bone mineral density or increased mineral crystal size reduces tissue elastic modulus and hardness. Specific Aim 1a: Mineral content will be reduced by 3 and 4 weeks of vitamin D-deficiency in growing rats. The treatment will be confirmed with whole bone bending tests. Tissue elastic modulus, composition and microstructure will be characterized. Tissue composition and microstructure will be correlated with indentation moduli and hardness in the femur and lumbar vertebra. Specific Aim 1b: Altered mineralization will be produced by 3 and 4 weeks of fluoride treatment of growing rats to weaken whole bone structure and alter mineral crystal size. Tissue properties will be analyzed and correlated as in Specific Aim 1 a. Hypothesis 2: Metabolic acidosis produces matrix changes that reduce tissue elastic modulus. Specific Aim 2: Metabolic acidosis in sheep is a preclinical model of human osteoporosis that reduces bone mass and alters tissue mineralization. The tissue properties will be analyzed and correlated in this sheep model as in Specific Aim 1. Additionally, bulk properties will be determined for specimens from the cortex. Models incorporating microstructural detail will be used to relate the behavior across length scales. Hypothesis 3: Bisphosphonate treatment increases tissue elastic modulus and hardness. Specific Aim 3: Bisphosphonate treatment will be used to overcome osteoporosis following metabolic acidosis from Specific Aim 2. Tissue and bulk properties will be analyzed and correlated as in Specific Aim 2. We will relate composition and microstructure of bone tissue to the elastic behavior, focusing on the role of mineral and collagen content, crystal size and collagen alignment. Our approach will cross-correlate several microlevel techniques that have not been previously used to characterize bone tissue mechanics.